Infection with influenza virus is known to increase susceptibility to bacterial infections of the respiratory tract. In a mouse model of influenza, increased bacterial colonization was also observed after administration of an infectious, attenuated influenza virus vaccine. Primary influenza virus infection increases colonization of the human upper and lower respiratory tract with bacteria, including Streptococcus pneumoniae and Staphylococcus aureus. Such infections may lead to complications of influenza, including pneumonia, bacteria in the blood, sinusitis, and ear infections.